Electrochromic element, materials for use in such element, processes for making such element and such materials and use of such element in an electrochromic glass device

ABSTRACT

An electrochromic element useful in an electrochromic glass or mirror device and a process for making such element. The element is a five-layered structure including an electrolyte ion conducting layer interposed between first and second inorganic electrochromic layers which are interposed between a pair of conductive electrodes. The first and second inorganic electrochromic layers are different and are capable of exhibiting color-forming properties complementary to one another upon the incorporation of an alkali metal or Ag ion, a mixture of alkali metal or Ag ions or a mixture of alkali metal or Ag and hydrogen ions. The electrolyte ion conducting layer may be a copolymer of ethylene oxide, butylene oxide or methyl glycidyl ether, and optionally a small amount of allyl glycidyl ether, along with an ionizable salt, or may be a polyurethane gel formed by reacting the copolymer with triisocyanate, along with an ionizable salt. The second inorganic electrochromic layer comprises a transition metal oxide which exhibits a color change when shifting between the +2 and +3 valence states. The second inorganic electrochromic layer may be produced by an electrochemical process, a chemical process, or by a physical process. The electrochromic element may also comprise a plurality of five-layer structures in tandem, each pair separated by a substrate.

This is a division of application Ser. No. 07/379,225, filed Jul. 13,1989, now U.S. Pat. No. 5,086,351.

BACKGROUND OF THE INVENTION

The present invention relates to electrochromic elements, and moreparticularly to laminated electrochromic glass devices and processes formaking such devices.

Electrochromic materials generally are materials which change color uponapplication of electrical current to induce an electrochemical reactionin the material.

Electrochromic devices are known which comprise a laminated structureincluding an electrolyte ion conducting layer sandwiched between anelectrochromic layer and a counter electrode layer, all sandwichedbetween a pair of conductive electrodes composed of, for example,indium-tin oxide.

Many of the prior art electrochromic elements utilize WO₃ as theelectrochromic color-forming layer. It is known that WO₃ changes from aclear, transparent state to a colored state upon undergoing thereaction: ##STR1## wherein Z is selected from H or alkali metals such asLi or Na.

It is also known from the nonaqueous secondary battery art that variousother metals will display electrochromic properties when changing fromone valence state to another. Specifically, it is known that sometransition metals exhibit electrochromic properties when moving betweenthe +2 and +3 valence states and other transition metals exhibit suchproperties when changing between the +3 and +4 valence states.

Heretofore, the art has had difficulty utilizing the electrochromicproperties of WO₃ in combination with the electrochromic properties ofthese other known transition metal oxides. For example, it is disclosedby U.S. Pat. No. 4,750,816 that "oxidatively color-forming materialssuitable as opposing electrodes of reductive color-forming layerscomprising WO₃, etc. are not found in inorganic materials." Column 1,lines 42-45). This is due to several factors, such as the difficulty indiscovering oxidative color-forming materials which (1) have a highenough ion exchange capacity; (2) exhibit an adequate transparency or,even better, exhibit color changes complementary to those of WO₃ ; and(3) have a range of working potential compatible with that of the othermaterials in the electrochromic element.

The term "complementary" color changes, as used herein, means exhibitingopposite color-forming tendencies upon the insertion of an ion. WO₃colors when an ion is inserted therein and thus materials"complementary" to WO₃ would bleach upon ion insertion. Thus, anelectrochromic element utilizing a layer of WO₃ along with a layer of amaterial having a complementary color change to WO₃ would have twobleached electrochromic layers when an ion was inserted into thecomplementary layer and two colored layers when an ion was inserted intothe WO₃ layer. This would enable an additive color effect to beattained.

Because of the aforementioned difficulties, prior art electrochromicdevices tended to utilize either a single electrochromic layer of WO₃ orother electrochromic material to produce the desired electrochromiccolor change effect, or utilized either an inorganic compound whichundergoes little or no color change upon ion insertion and removal or anorganic compound as the opposing or counter electrode to the WO₃ layer.The use of a single electrochromic layer of WO₃ or a layer of WO₃ inconjunction with a counter electrode which remains transparent upon ioninsertion and removal, suffers from the disadvantage that the differencein the amount of light that is transmitted through the layer in theclear and colored states is limited by the extent of color change of theWO₃ material. In addition, electrochromic devices utilizing an organicelectrochromic layer suffer from the disadvantage that these layers areunstable over long periods of time and thus their long termcolor-forming durability is questionable.

It is an object of the present invention to provide a novelelectrochromic element.

It is another object of the present invention to provide a novelelectrochromic glass device.

It is another object of the present invention to provide anelectrochromic element which is characterized by having a largedifference between the percentage of visible light transmitted by theelement in the colored state and the percentage of visible lighttransmitted by the element in the bleached state.

It is another object of the present invention to provide anelectrochromic element which is characterized by having a largedifference between the percentage of radiant heat transmitted by theelement in the colored state and the percentage of radiant heattransmitted by the element in the bleached state.

It is another object of the present invention to provide anelectrochromic element whose color-forming properties exhibit excellentlong term durability.

It is another object of the present invention to provide anelectrochromic element characterized by having an excellent responsetime, i.e., the period of time to change between the bleached state andthe colored state is low.

It is another object of the invention to provide an electrochromicelement that can operate effectively over a wide range of temperatures.

It is a further object of the invention to provide an electrochromicelement that does not utilize toxic or corrosive materials.

Additional objects and advantages of the invention will be set forth inpart in the description which follows, and in part will be obvious fromthe description, or may be learned by practice of the invention.

SUMMARY OF THE INVENTION

As embodied and broadly described herein, the electrochromic element ofthe present invention can be a five-layered structure which comprises apair of electrodes at least one of which is transparent, first andsecond inorganic electrochromic layers interposed between the pair ofelectrodes and an ion conducting layer of an electrolyte interposedbetween the first and second inorganic electrochromic layers. The firstand second inorganic electrochromic layers are preferably composed ofdifferent materials each of which is capable of exhibitingelectrochromic properties upon the incorporation of an alkali metal orAg ion. In addition, the electrochromic properties of the first andsecond inorganic electrochromic layers are preferably complementary toone another.

The electrochromic element of the invention, as embodied and broadlydescribed herein, more particularly has a first inorganic electrochromiclayer which is preferably WO₃ and a second inorganic electrochromiclayer preferably comprising a transition metal oxide which exhibits acolor change when shifting between the +2 and +3 valence states.

In another aspect of the invention, as embodied and broadly describedherein, an electrochromic material is provided which may be utilized asthe second inorganic electrochromic layer in the electrochromic elementof the invention. This material comprises a transition metal oxide whichexhibits a color change when shifting between the +2 and +3 valencestates and can be represented by the formula M_(x+y) T^(II) _(1-2x+y)T^(III) _(x-y) O. M is selected from an alkali metal or Ag, an alkalimetal or Ag and hydrogen, and a mixture of alkali or Ag metals. T^(II)and T^(III) are divalent and trivalent forms of a transition metal T. xis the mole fraction of M⁺ ions that are incorporated into theelectrochromic material in a first stage, y is the mole fraction of M⁺ions that are incorporated into the electrochromic material in a secondstage, 0<x≦1.0, -x≦y≦+x, y≧2x-1 and the electrochromic materialundergoes a maximum transmissivity change when y varies between - x and+x.

The electrochromic element of the invention utilizing the aboveelectrochromic material as the second inorganic electrochromic layer iscapable of exhibiting a first color state when y=x wherein the firstinorganic electrochromic layer is bleached and has the composition WO₃and the second inorganic electrochromic layer is bleached and has thecomposition M_(2x) T^(II) _(1-x) O, and a second color state when y<xwherein the first inorganic electrochromic layer is colored and has thecomposition MWO₃ and the second inorganic electrochromic layer iscolored and has the composition M_(x+y) T^(II) _(1-2x+y) T^(III) _(x-y)O. The first color state has a maximum transmissivity and the secondcolor state has a less than maximum transmissivity. The minimumtransmissivity would be attained when y=-x.

In another aspect of the invention, as embodied and broadly describedherein, an electrochromic element is provided which comprises aplurality of five-layered electrochromic elements as described above,positioned in juxtaposed surface to surface relation with one another,each pair of five-layered elements separated by a substrate layer. Morespecifically, a device comprising a pair of five-layered elementscomprises the following layers, in order: a first transparent substrate;a first transparent conductive electrode; a first inorganicelectrochromic layer; a first ion conducting layer of an electrolyte; afirst inorganic electrochromic counter electrode layer; a secondtransparent conductive electrode; a second transparent substrate; athird transparent conductive electrode; a second inorganicelectrochromic counter electrode layer; a second ion conducting layer ofan electrolyte; a second inorganic electrochromic layer; a fourthconductive electrode which may be transparent or reflective; and a thirdtransparent substrate. The first and second inorganic electrochromiclayers are preferably different from the first and second inorganicelectrochromic counter electrode layers. The first and second inorganicelectrochromic layers and the first and second inorganic electrochromiccounter electrode layers are preferably capable of exhibitingelectrochromic properties upon the incorporation of an alkali metal orAg ion. In addition, the electrochromic properties of the first andsecond inorganic electrochromic layers preferably are complementary tothe electrochromic properties of the first and second inorganicelectrochromic counter electrode layers.

The invention also relates to processes for making the electrochromicmaterial described above. As embodied and broadly described herein, onesuch process for producing an electrochromic material comprises:

a first step of forming an original thin film consisting essentially ofTO, T(OH)₂ and TOOH by sputtering a target comprising T with O₂ /H₂plasma;

a second step of electrochemically processing the original thin film inalkali metal hydroxide solution to give a layer consisting essentiallyof TO and TOOH;

a third step of electrochemically processing the layer consistingessentially of TO and TOOH in a liquid electrolyte comprising a polarsolvent selected from propylene carbonate and (C₂ H₅)₂ NSO₂ N(C₂ H₅)₂and an ionizable salt MZ, wherein M is an alkali metal or Ag and Z is astrong acid anion selected from ClO₄ ⁻, CF₃ SO₃ ⁻ and N(CF₃ SO₂)₂ ⁻, toincorporate xM⁺ ions in a first stage and yM⁺ ions in a second stageinto the layer to form an electrochromic material having the compositionM_(x+y) T^(II) _(1-2x+y) T^(III) _(x-y) O;

wherein T is a transition metal selected from Ni, Co, Mn and Fe; M isselected from an alkali metal or Ag, an alkali metal or Ag and hydrogenand a mixture of alkali or Ag metals; and T^(II) and T^(III) aredivalent and trivalent forms of transition metal T.

Another process for producing an electrochromic material, as embodiedand broadly described herein, comprises:

a first step of sputtering a target of --M.sub.α T₁₋α O --; wherein0<α≦1.0, to form a thin film of M_(m) T_(n) ^(II) T_(p) ^(III) O;

a second step of electrochemically processing the thin film in a liquidelectrolyte comprising a polar solvent selected from propylene carbonateand (C₂ H₅)₂ NSO₂ N(C₂ H₅)₂ and an ionizable salt MZ, wherein M is analkali or Ag metal and Z is a strong acid anion selected from ClO₄ ⁻,CF₃ SO₃ ⁻ and N(CF₃ SO₂)₂ ⁻, to incorporate xM⁺ ions in a first stageand yM⁺ ions in a second stage in the thin film to form anelectrochromic material having the composition M_(x+y) T^(II) _(1-2x+y)T^(III) _(x-y) O;

wherein T is a transition metal selected from Ni, Co, Mn and Fe; T^(II)and T^(III) are divalent and trivalent forms of transition metal T; M isselected from an alkali metal or Ag, an alkali metal or Ag and hydrogen,and a mixture of alkali or Ag metals; and m, n and p are mole fractions.

The invention also relates to a method of manufacturing theelectrochromic device as described above. This method, as embodied andbroadly described herein, preferably comprises:

sputtering a layer of a conductive electrode material on a firstsubstrate;

sputtering a layer of a first inorganic electrochromic material onto theconductive electrode layer on the first substrate;

sputtering a layer of a conductive electrode material on a secondsubstrate;

sputtering a layer of a second inorganic electrochromic material ontothe conductive electrode layer on the second substrate; and

assembling an ion conducting layer of an electrolyte between thesputtered sides of the first and second substrates.

The invention further relates to a method of manufacturing theelectrochromic device comprising a pair of five-layered electrochromicelements in back to back relation. Such a method, as embodied andbroadly described herein, comprises:

sputtering a layer of a conductive electrode material on a firstsubstrate;

sputtering a layer of a first inorganic electrochromic material onto theconductive electrode layer on the first substrate;

sputtering a layer of a conductive electrode material on a secondsubstrate;

sputtering a layer of a first inorganic electrochromic material onto theconductive electrode layer on the second substrate;

sputtering a layer of a conductive electrode material on each side of athird substrate; and

sputtering a layer of a second inorganic electrochromic material on theconductive electrode layer on each side of the third substrate. Thesputtered side of the first substrate is assembled into juxtaposedcontact with one side of the third substrate with an ion conductinglayer of an electrolyte interposed therebetween. The sputtered side ofthe second substrate is then assembled into juxtaposed contact with theother side of the third substrate with an ion conducting layer of anelectrolyte interposed therebetween.

The accompanying drawings, which are incorporated in and constitute apart of this specification, illustrate preferred embodiments of theinvention and together with the detailed description of the preferredembodiments herein, serve to explain the principles of the invention.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a sectional view showing a construction of one embodiment ofthe laminate electrochromic device of the present invention.

FIG. 2 is a sectional view showing a construction of a second embodimentof the laminate electrochromic device of the invention.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

Reference will now be made in detail to the present preferredembodiments of the invention, examples of which are illustrated in theaccompanying drawings.

In accordance with the present invention, there is provided anelectrochromic element comprising a pair of electrodes at least one ofwhich is transparent, first and second inorganic electrochromic layersinterposed between the pair of conductive electrodes and an ionconducting layer of an electrolyte interposed between the first andsecond inorganic electrochromic layers, wherein the first and secondinorganic electrochromic layers are different and are capable ofexhibiting electrochromic properties upon the incorporation of an alkalimetal or Ag ion and wherein the electrochromic properties of the firstand second inorganic electrochromic layers are complementary. Asembodied in FIG. 1, electrochromic element 10 comprises a pair ofconductive electrodes 12 and 14. Conductive electrodes 12 and 14 mayboth be transparent or one may be transparent and the other reflective.

In a preferred embodiment of the invention, electrochromic element 10 isutilized in an electrochromic glass device. In such an embodiment,conductive electrodes 12 and 14 are both transparent and are formed ontwo substrates 22 and 24 made of, for example, glass or plastic.Conductive electrodes 12 and 14 may be any of those materials known inthe art to be useful as transparent conductive electrodes and arepreferably composed of indium tin oxide, which is a composition of In₂O₃ containing 5 wt. % of SnO₂, or fluorine doped tin oxide (SnO₂ :F).When utilizing indium tin oxide as conductive electrodes 12 and 14, theindium tin oxide is preferably deposited on float glass. Pyrrolyticglass (SnO₂ :F) as supplied by the glass industry may also be utilized,which would function as both conductive electrodes 12 and 14 andsubstrates 22 and 24.

In another embodiment of the invention, electrochromic element 10 isutilized in a mirror. In such an embodiment, one of conductiveelectrodes 12 and 14 is reflective and the other is transparent. Theconductive electrode 12 or 14 that is reflective may be any of thosematerials known in the art to be useful as reflective conductiveelectrodes, such as Al, Au or Ag.

In accordance with the present invention as embodied in FIG. 1, firstinorganic electrochromic layer 16 and second inorganic electrochromiclayer 18 are interposed between conductive electrodes 12 and 14. Firstinorganic electrochromic layer 16 preferably comprises WO₃. WO₃ isbleached in its normal state and is thus transparent. However, thecompound WO₃ has the property that it exhibits a colored state when H⁺ions, Ag⁺ ions or alkali metal ions such as Li⁺ are incorporatedtherein. First inorganic electrochromic layer 16 may alternativelycomprise CrO₂, FeO or MoO₃, or any of those compounds known in the artthat are bleached in the normal state and colored when H⁺, Ag⁺ or alkalimetal ions are incorporated therein.

In accordance with the invention, first inorganic electrochromic layer16 may be produced by sputtering a tungsten target to form a film of WO₃or alternatively, the film of WO₃ may be electrochemically processed.

In accordance with the invention, second inorganic electrochromic layer18 preferably comprises a transition metal oxide having the property ofexhibiting a color change when shifting between the +2 to +3 valencestates. The composition of second inorganic electrochromic layer 18 canbe represented by the formula M_(x+y) T^(II) _(1-2x+y) T^(III) _(x-y) O.T^(II) and T^(III) are divalent and trivalent forms of transition metalT, and M is selected from an alkali metal or Ag, an alkali metal or Agand hydrogen, and a mixture of alkali or Ag metals. The transition metalT that can be utilized in second inorganic electrochromic layer 18includes Ni, Co, Mn and Fe. The alkali metal utilized in secondinorganic electrochromic layer 18 may be Li, Na, K or Cs and ispreferably Li. x represents the mole fraction of M⁺ ions that areincorporated into second inorganic electrochromic layer 18 in a firststage. y represents the mole fraction of M⁺ ions that are incorporatedinto second inorganic electrochromic layer 18 in a second stage.

In accordance with the invention, alkali metal or Ag ions can beincorporated into first and second inorganic electrochromic layers 16and 18 without the corresponding incorporation of H⁺ ions. However,during assembly of electrochromic element 10, it is possible that someamount of water may enter the system. The presence of water in theelectrochromic system may cause certain amounts of H⁺ ions to be formedwhich will inevitably become incorporated into first and secondinorganic electrochromic layers 16 and 18 along with alkali metal or Agions. The presence of H⁺ ions in the system can cause problems in thatWO₃ degrades in the presence of H₂ O, which may be formed as aby-product. In addition, H₂ gas may be formed as a by-product from H⁺ions and can cause bubble formation (often called "outgassing"). On theother hand, H⁺ ions are smaller than alkali metal or Ag ions and thusare more mobile which results in faster incorporation and extractionfrom first and second inorganic electrochromic layers 16 and 18. Amixture of alkali metal or Ag ions and hydrogen ions incorporated intofirst and second inorganic electrochromic layers 16 and 18 will havesome of the characteristics of both H⁺ incorporation and M⁺incorporation.

In accordance with the invention, second inorganic electrochromic layer18 may be produced by an electrochemical method of first sputtering atarget comprised of a transition metal T with a plasma of O₂ /H₂ to forman original thin film layer consisting essentially of a mixture of TO,T(OH)₂ and TOOH. This original layer is then preferablyelectrochemically processed in an alkali metal hydroxide solution, forinstance NaOH, 1N solution, wherein the original layer is the cathodeand the anode is a platinum electrode. This electrochemical step yieldsa layer consisting essentially of TO and TOOH. The resulting layer isthen electrochemically processed in a liquid electrolyte solution withone electrode consisting of the TO/TOOH mixture with an opposing lithiumelectrode. The liquid electrolyte may comprise a polar solvent selectedfrom propylene carbonate and (C₂ H₅)₂ NSO₂ N(C₂ H₅)₂ and an ionizablesalt MZ, wherein M is an alkali metal or Ag and Z is a strong acid anionselected from ClO₄ ⁻, CF₃ SO₃ ⁻ and N(CF₃ SO₂)₂ ⁻. The electrochemicalreaction that takes place causes a particular mole fraction of M⁺ ions,designated herein as x, to become incorporated into the layer in a firststage. This electrochemical reaction is designated by the followingequation:

    [(1-2x)TO,2xTOOH]+xM.sup.+ +xe.sup.- →[(1-2x)TO,xMTO.sub.2, xHTOOH]

While not intending to be bound by any theory of how the inventionworks, it is believed that the H atoms in TOOH are loosely attached andmay migrate into the interior of the layer. It is believed that a solidsolution is spontaneously formed which comprises M_(x) T^(II) _(1-2x)T^(III) _(x) O.

In addition to the M⁺ ions that can be incorporated into the layer in afirst stage, yM⁺ additional ions can be incorporated in a second stageby the following equation:

    M.sub.x T.sup.II.sub.1-2x T.sup.III.sub.x +yM.sup.+ +ye.sup.- →M.sub.x+y T.sup.II.sub.1-2x+y T.sup.III.sub.x-y O

wherein y is the mole fraction of M⁺ ions that are incorporated into thelayer in this second stage.

The present invention utilizes the property of transition metals such asNi, Co, Mn and Fe that undergo a color change when moving between the +2and +3 valence states. Generally, these transition metals are colored inthe +3 valence state and bleached in the +2 valence state. It can beseen that if y=-x, representing the state of no incorporation, then thelayer will consist of M_(x) T^(II) _(1-3x) T^(III) _(2x) O, which willbe colored because of the presence of T^(III). However, if y=y_(max) =x,which represents the ion incorporated state, then the layer will consistof M_(2x) T^(II) _(1-x) O. The layer will thus be bleached because of anabsence of T^(III).

A preferred result could be achieved if x=y_(max) =0.5, wherein theequation representing reversible incorporation would be M₀.5 T₀.5^(III)O+(0.5)M⁺ +(0.5)e⁻ →MT₀.5^(II) O. In this case, the reversibleincorporation of M⁺ ions would cause the electrochromic layer to go froma compound having T entirely in the +3 transition state to a compoundhaving T entirely in the +2 transition state, which would represent themaximum color change.

In accordance with the invention, an alternative method for producingsecond inorganic electrochromic layer 18 is a physical preparation whichcomprises a first step of sputtering a target of M.sub.α T₁₋α O, whereinO<x≦1.0 and α represents mole fraction, to form a thin film of M_(m)T_(n) ^(II) T_(p) ^(III) O, wherein m, n and p represent mole fractionsof M, T^(II) and T^(III), respectively; and a second step ofelectrochemically processing the thin film in a liquid electrolytecomprising a polar solvent selected from propylene carbonate and (C₂H₅)₂ NSO₂ N(C₂ H₅)₂ and an ionizable salt MZ, wherein M is an alkalimetal or Ag and Z is a strong acid anion selected from ClO₄ ⁻, CF₃ SO₃ ⁻and N(CF₃ SO₂)₂ ⁻, to incorporate xM⁺ ions in a first stage andincorporate yM⁺ ions in a second stage into the thin film to form theelectrochromic material having the composition M_(x+y) T^(II) _(1-2x+y)T^(III) _(x-y) O.

In accordance with the invention, an alternative method for producingeither first or second inorganic electrochromic layers 16 or 18 is amethod that consists of chemically inserting alkali metal atoms such aslithium or silver atoms into either layer. This method overcomes thepotential problem of H⁺ contamination since protons are incapable ofco-existing with alkali metal or Ag in these solutions.

In accordance with the present invention as embodied in FIG. 1, ionconducting layer 20 is interposed between first and second inorganicelectrochromic layers 16 and 18. Ion conducting layer 20 preferablyconsists of a solid polymeric electrolyte, which is an amorphous solidsolution comprising a copolymer of ethylene oxide and methyl glycidylether and at least one ionizable salt. Alternatively, the copolymer maybe ethylene oxide and butylene oxide. The preferred proportions of thecopolymer are 75% ethylene oxide and 25% methyl glycidyl ether orbutylene oxide. In addition, a small amount of allyl glycidyl ether (5%)may be included in the copolymer. The molecular weight of the copolymerpreferably ranges between 30,000 and 2,000,000. The ionizable saltutilized in conjunction with the solid copolymer, can be a mixture ofMClO₄ and MN(CF₃ SO₂)₂ or can consist entirely of MN(CF₃ SO₂)₂, whereinM is an alkali metal or Ag, preferably Li. The solid polymericelectrolyte may also include a plasticizer such as (C₂ H₅)₂ NSO₂ N(C₂H₅)₂.

In accordance with an alternative embodiment of the invention, ionconducting layer 20 may comprise a polyurethane made by reactingtriisocyanates with the above-mentioned copolymers having a lowmolecular weight (1,000-20,000) along with at least one of theabove-mentioned ionizable salts. Such a polyurethane network utilized inion conducting layer 20 chemically crosslinks and hardens at roomtemperature.

The basic chemical reaction is: ##STR2## These polyurethane networkshave the advantage of nearly perfect optical transparency.

In accordance with the invention, the ion conducting layer 20 may alsoutilize a liquid electrolyte such as LiClO₄ -propylene carbonate.However, a solid polymeric electrolyte for use in ion conducting layer20 is preferable over liquid electrolytes because the solid polymers aremuch easier to handle in assembling the electrochromic device andpresent far fewer safety concerns in the assembled device from potentialleaking. One important factor which can cause "haze" problems inelectrochromic elements is the crystallization of the ion conductinglayer. The solid polymeric electrolyte of the invention comprising acopolymer of ethylene oxide and methyl glycidyl ether or butylene oxidealong with at least one ionizable salt and the solid polymericelectrolyte comprising a polyurethane gel and at least one ionizablesalt each provides an efficient layer for conducting ions between firstand second inorganic electrochromic layers 16 and 18 without significantcrystallization of the solid polymer electrolyte, i.e., the solidpolymer electrolyte remains amorphous. In addition, the copolymerincluding butylene oxide has the advantage of being less hydrophilic.

The ion conducting macromolecular material of the present invention canbroadly be any polymer-based material exhibiting an ionic conductivityat least equal to 10⁻⁷ siemens/cm at room temperature and an electronicconductivity lower than 10⁻¹⁰ siemens/cm.

In particular, the ion conducting macromolecular material can comprise asolid solution of at least one ionizable salt, especially an alkalimetal or Ag salt and more particularly a lithium salt, in a plasticpolymeric material comprising at least in part one or more polymersand/or copolymers of monomers containing at least one heteroatom,especially oxygen or nitrogen, able to form donor/acceptor bonds withthe cation of the ionizable salt, the polymers being in particular,chosen among polyethers, and more especially among the homopolymers ofethylene oxide or propylene oxide (see European Patent Application No.0013199). The plastic polymeric material can comprise a copolymer ofethylene oxide and of another cyclic oxide, said copolymer having eitherthe structure of a statistical copolymer (U.S. Pat. No. 4,578,326) whichmay be crosslinked (French Patent No. 2,570,224) or the form of apolyurethane network resulting from the reaction of a sequencedcopolymer of ethylene oxide and another cyclic oxide with a couplingagent consisting of an organic polyisocyanate (French Patent No.2,485,274). Moreover, the ionizable salts mentioned in European PatentApplication No. 0013199 can be replaced in whole or in part by ionizablesalts such as alkali metal closoboranes (French Patent No. 2,523,770),alkali metal tetrakis-trialkylsiloxyalanates (French Patent No.2,527,611), alkali metal bis(perhalogenoalkylsulfonyl)imidides orbis(perhalogenoacyl)imidides (French Patent No. 2,527,602), alkali metaltetraalkynylborates or aluminates (French Patent No. 2,527,610), alkalimetal derivatives of perhalogenoalkylsulfonylmethane orperhalogenoacetylmethane (French Patent No. 2,606,218), or alkali metalsalts of polyethoxylated anions (European Patent Application No.0,213,985).

The ion conducting macromolecular material of the invention can alsobroadly consist of a solid solution of an ionizable salt, for exampleone of those mentioned above, in a polymeric material made up with anorganometallic polymer in which at least two polyether chains are linkedby a metallic atom selected from Al, Zn and Mg (French Patent No.2,557,735) or among Si, Cd, B and Ti (French Patent No. 2,565,413), orin a polymeric material consisting of a polyphosphazene bearing on eachphosphorus atom two polyether groups such as polyethylene oxide groups.The ion conducting macromolecular material may also be selected from themixtures of polymers having a solvating and/or polar character with anysalt, acid or base sufficiently dissociated in the polymer to obtain theappropriate conductivity, from polymers bearing ionizable functionsresulting in anions or cations attached to the macromolecular chains,from protonic conductors such as those described in French Patent No.2,593,328 or mixtures of inert polymers with mineral or organic ionconducting materials dispersed in the polymer matrix.

In a preferred embodiment of the present invention, electrochromicelement 10 is interposed between a pair of glass or plastic substrates22 and 24. Such an arrangement forms an electrochromic device. Theelectrochromic device can be manufactured by sputtering on a glass oplastic substrate 22 or 24, conductive electrode 12 which may becomposed of indium-tin oxide or fluorine doped tin oxide (SnO₂ :F).First inorganic electrochromic layer 16 is then sputtered ontoconductive electrode 12. On a second glass or plastic substrate 22 or24, conductive electrode 14 is sputtered, and second inorganicelectrochromic layer 18 is sputtered onto conductive electrode 14. Thetwo sputtered glass substrates are then assembled with ion conductinglayer 20, which may be a solid polymeric electrolyte, interposedtherebetween.

In accordance with the invention, it should also be possible tomanufacture electrochromic element 10 by depositing all of the activelayers, i.e., conductive electrodes 12 and 14, first and secondinorganic electrochromic layers 16 and 18 and ion conducting layer 20 inthe form of a gel. (See Solid State Ionics 28-30 (1988)-1722).

In accordance with the invention, M⁺ ions can be incorporated into firstinorganic electrochromic layer 16 prior to being assembled in theelectrochromic element 10. Alternatively, M⁺ ions can be incorporatedinto second inorganic electrochromic layer 18 prior to assembly into theelectrochromic device. In either case, the application of a voltagedifferential between conductive electrodes 12 and 14 will cause the M⁺ion to move out of one inorganic electrochromic layer 16 or 18, throughion conducting layer 20 and into the other inorganic electrochromiclayer 16 or 18, thereby causing each of first and second inorganicelectrochromic layers 16 and 18 to become either bleached or colored.

In accordance with the invention, the voltage differential betweenconductive electrodes 12 and 14 sufficient to cause M⁺ ions to beincorporated into either first or second inorganic electrochromic layers16 and 18 is less than or equal to 3.5 volts vs. Li. This makes firstand second inorganic electrochromic layers 16 and 18 compatible with ionconducting layer 20 when utilizing a solid polymeric electrolytecontaining a lithium salt which will decompose at potentials greaterthan or equal to 3.5 volts vs. Li.

In an alternative embodiment of the invention, as embodied in FIG. 2,electrochromic element 28 comprises:

a first transparent conductive electrode 30 which may be indium tinoxide or fluorine doped tin oxide;

first inorganic electrochromic layer 32 which may be WO₃, MoO₃ or CrO₂ ;

first ion conducting layer of an electrolyte 34 which may be a solidpolymeric electrolyte comprising a terpolymer of ethylene oxide,butylene oxide and allyl glycidyl ether and at least one ionizable saltor may be a solid polymeric electrolyte comprising a polyurethane geland at least one ionizable salt;

first inorganic electrochromic counter-electrode layer 36 which may bethe same transition metal oxides as disclosed earlier for secondinorganic electrochromic layer 18 of FIG. 1;

second transparent conductive electrode 38 which may be indium tin oxideor fluorine doped tin oxide;

transparent substrate 40 which may be glass or a plastic;

third transparent conductive electrode 42 which may be indium tin oxideor fluorine doped tin oxide;

second inorganic electrochromic counter-electrode layer 44 which may bethe same transition metal oxides as disclosed earlier for secondinorganic electrochromic layer 18 of FIG. 1;

second ion conducting layer of an electrolyte 46 which may be the samematerials as for layer 34;

second inorganic electrochromic layer 48 which may be WO₃, MoO₃ or CrO₂;

and fourth conductive electrode 50 which may be transparent orreflective.

First and second inorganic electrochromic layers 32 and 48 arepreferably different from first and second inorganic electrochromiccounter-electrode layers 36 and 44. First and second inorganicelectrochromic layers 32 and 48 and first and second inorganicelectrochromic counter-electrode layers 36 and 44 are preferably capableof exhibiting electrochromic properties upon the incorporation of analkali metal or Ag ion. The electrochromic properties of first andsecond inorganic electrochromic layers 32 and 48 are preferablycomplementary to the electrochromic properties of first and secondinorganic electrochromic counter-electrode layers 36 and 44. Within thescope of the present invention, electrochromic element 28 may beinterposed between two layers of transparent substrate materials 22 and24, such as glass or plastic.

In accordance with the invention, electrochromic element 28 can bemanufactured by sputtering a layer of a conductive electrode material ona first transparent substrate 22 or 24 to form first transparentconductive electrode 30 followed by sputtering a layer of a firstinorganic electrochromic material onto first transparent conductiveelectrode 30 to form first inorganic electrochromic layer 32. Similarly,fourth conductive electrode 50 and second inorganic electrochromic layer48 can be formed on a second glass substrate 22 or 24. A third glasssubstrate 40 can be sputtered with second transparent conductiveelectrode 38 and first inorganic electrochromic counter-electrode layer36 on one side and sputtered with third transparent conductive electrode42 and second inorganic electrochromic counter-electrode layer 44 on theother side. First ion conducting layer of an electrolyte 34 can beassembled between the sputtered side of first glass substrate 22 or 24and either side of third glass substrate 40. Second ion conducting layerof an electrolyte 46 can then be assembled between the sputtered side ofsecond glass substrate 22 or 24 and the other side of third glasssubstrate 40 to form electrochromic element 28.

The electrochromic element of the present invention utilizing Ni as thetransition metal in second inorganic electrochromic layer 18 and Li ionsas the insertion ions has achieved an ion exchange of 5-6 mC/cm². At ionexchanges greater than 5-6mC/cm², durability of the element begins todecline. It is believed that this decline in durability is caused byirreversible damage at the conductive electrode/counter-electrodeinterface. The electrochromic element of the present invention has beenshown to be durable over more than 4,500 cycles in severe conditionswhile achieving complete bleaching and coloration at each cycle. Changesin transmissivity of the electrochromic element of the invention havebeen achieved ranging from 30-35% to approximately 85% of visibletransmitted light when utilizing the element of FIG. 1 and also lowerranges have been achieved ranging from 3-5% to 55-60% of visibletransmitted light when utilizing the element of FIG. 2. The switchingtime of the electrochromic element of the invention, i.e., the time togo from the colored state to the bleached state is in the range of 5-10minutes.

The following Examples are provided to illustrate the present inventionand some of its advantages. The Examples are not intended to limit theinvention.

EXAMPLE 1 Manufacture of a Solid State Device Manufacture of TransparentConductive Electrodes (TE)

Transparent conductive electrodes consisting of ITO (Indium Tin Oxide),deposited by reactive DC sputtering from an indium tin target, weredeposited on float glass (5×5 cm²) under the following conditions:

    ______________________________________                                        Initial pressure:  10.sup.-5 mb                                               Oxygen pressure:   10.sup.-3 mb                                               Argon pressure:    2.2 10.sup.-3 mb                                           Total pressure:    3.2 10.sup.-3 mb                                           Power:             400 W                                                      Voltage:           515 V                                                      Sputtering time:   10 min.                                                    Annealing:         450° C. for 30 min.                                 Properties of the films:                                                      thickness = 1600 A                                                            sheet resistance R.sub.O = 50 ohms                                            optical transmission at 550 nm: 90%                                           Preparation of First Inorganic Electrochromic Layer EC.sub.1                  ______________________________________                                    

WO₃ was prepared by reactive DC sputtering from a tungsten target underthe following conditions:

    ______________________________________                                        Initial pressure:    10.sup.-5 mb                                             Oxygen pressure:     8 × 10.sup.-3 mb                                   Power:               1000 W                                                   Voltage:             490 V                                                    Sputtering time:     50 min.                                                  ______________________________________                                    

The WO₃ films thereby obtained can be either directly used orelectrochemically processed ("formated") in H₂ SO₄, 1N solution prior toutilization. A three electrode cell configuration was used for thisprocessing: the electrochromic material EC₁, a platinum counterelectrode and a saturated calomel reference electrode (SCE). Theelectrochemical treatment consisted first of a cathodic polarization ofEC₁ at 0.5 V vs. SCE for 120 seconds then followed by an anodicpolarization at +0.5 V vs. SCE for 120 seconds. This cycle was repeatedthree times and the procedure was terminated with the anodicpolarization. Finally, the films were rinsed in distilled water and thendried at room temperature.

The performances of both types of thin films (straight from sputteringand formated in H₂ SO₄) are compared in the following table(transmission measurements were carried out at 550 nm).

    __________________________________________________________________________    Thickness   Exchanged charge                                                                       Transm. colored                                                                        Transm. bleached                                                                       Coloring time                                                                        Bleaching time                  (A)         (mC/cm2) (%)      (%)      (min.) (min.)                          __________________________________________________________________________    WO.sub.3                                                                            3000  7        45       90       6      5                               (straight)                                                                    WO.sub.3                                                                            3000  10       35       90       4      3                               (formated)                                                                    Preparation of Second Inorganic Electrochromic Layer (EC.sub.2)                                              electrochemcial                                               Two methods have been used:                                                                  physical                                        __________________________________________________________________________

Electrochemical preparation involved three steps

1st step

The "original" layer was prepared by reactive DC sputtering from anickel target under the following conditions:

    ______________________________________                                        Initial pressure:                                                                         10.sup.-5 mb                                                                              Power:      300 W                                     Oxygen pressure:                                                                          7.2 × 10.sup.-3 mb                                                                  Voltage     240 V                                     Hydrogen pressure:                                                                        0.4 × 10.sup.-3 mb                                                                  Sputtering time:                                                                          60 min.                                   Total pressure:                                                                           7.6 × 10.sup.-3 mb                                                                  Film thickness:                                                                           1100 A                                    ______________________________________                                    

This produced a thin film consisting of a mixture of NiO, Ni(OH)₂ andNiOOH.

2nd step

After sputtering, the film was electrochemically processed in NaOH, 1N,in a manner similar to that described above for WO₃, but with an anodicpolarization vs SCE for 2 min.

This yielded the formation of a layer of NiOOH and NiO. Once formated,the film was rinsed in distilled water, then dried at room temperature.

3rd step

The final active material, namely Li_(x+y) Ni^(II) _(1-2x+y) Ni^(III)_(x-y) O, was obtained after an electrochemical treatment performed in adry box. The procedure utilized a two-electrode cell configuration,namely the Ni based film and a lithium electrode; both electrodes wereimmersed in LiClO₄ (1M)propylene carbonate (LiClO₄ p.c.). The film wasthen polarized at 1.7 V vs Li for 60 min. to produce the above mentionedactive material Li_(x+y) Ni^(II) _(1-2x+y) Ni^(III) _(x-y) O.

Physical preparation

Steps 1 and 2 above were replaced by reactive direct sputtering (RF)from a Li₀.3 Ni₀.7 O target having a 75 mm diameter.

Target manufacturing

A mixture (powder) of 0.15 Li₂ CO₃ +0.7 NiO (molar proportion) was firstheated in air at 1000° C. for 8 hours, and then compacted at 50 tons for10 mins. The material thereby obtained was finally sintered in air at1000° C. for 8 hours. The conditions for the reactive RF sputtering werethe following:

    ______________________________________                                        Initial pressure:                                                                         10.sup.-5 mb                                                                              Voltage:    200 V                                     Oxygen pressure:                                                                          2.5 × 10.sup.-2 mb                                                                  Sputtering time:                                                                          120 mn                                    Powder:     30 W        Film thickness:                                                                           1100 A                                    ______________________________________                                    

The thin film obtained by sputtering was then processed in LiClO₄ p.c.as the third step of the electrochemical method described above toproduce the final Li_(x+y) Ni_(1-x) O material.

The performance of both types of Ni based thin films are compared in thefollowing table (light transmission is measured at 550 nm).

    __________________________________________________________________________    Thickness   Exchanged charge                                                                       Transm. colored                                                                        Transm. bleached                                                                       Coloring time                                                                        Bleaching time                  (A)         (mC/cm2) (%)      (%)      (mn)   (mn)                            __________________________________________________________________________    Electro                                                                             1100   6       45       90       30     20                              chemical                                                                      preparation                                                                   Physical                                                                            1100  12       20       75        3      2                              preparation                                                                   __________________________________________________________________________

Preparation of the Solid Polymer Electroyle (SPE)

The solid polymer electrolyte was a "solid solution" of a lithium saltin a copolyether type polymer. The polymer was an ethylene oxide basedterpolymer comprised of: ##STR3##

An equimolar mixture of LiClO₄ and LiN(CF₃ SO₂)₂ salt was dissolved inthe polymer (15% wt.) to form the solid polymer electrolyte. Theincorporation of the salt into the polymer was operated in air by meansof a co-solvent like acetonitrile (CH₃ CN).

The above solution (polymer+salt in CH₃ CN) was spread (by adoctor-blade technique) onto the two electrodes (electrochromic films)at a thickness of 200 microns.

To remove the solvent and obtain a layer of solid polymer electrolyte oneach substrate, they were dried at 70° C. under 12 bars pressure minimum(in air). Each SPE layer obtained was 20 microns thick.

Assembly of the Complete Device

The two parts (glass+TE+EC) were then assembled together with the SPEinterposed between, in a vacuum press (0.5 mb). Prior to assembling,however, the parts were heated separately at 80° C. for 10 mins. and theair was removed out of the press chamber using a vacuum pump (0.5 mb).The parts were then pressed against each other at roughly 50 Kg/in² for3 mins.

Finally, the device was sealed (in air) with a low vapor pressure resinin order to prevent the contamination by air and moisture.

Performance of the Complete Device

A device prepared containing the electrochemically prepared Ni basedmaterial EC₂ was evaluated over a number of cycles. EC₁ was straight WO₃obtained by DC sputtering. The polymer-salt combination utilized as theion conducting layer was the terpolymer described above.

The characteristics of the device were the following:

surface area: 20 cm²

working potential range (WO₃ vs Ni based EC₂): -1.6 V for coloring; +1.4V for bleaching

charge (Li⁺) exchanged: 5-6 mC/cm²

number of cycles: over 2000

maximum transmission changes at 550 nm: 33%→85%

times to achieve the transmission changes:

    ______________________________________                                        Transmission %:   85    35     78  40   73  45                                Time (min.):       5    12      3   7    2   4                                % of total transmission change:                                                                 100%      90%     80%                                       ______________________________________                                    

EXAMPLE 2

The preparation of this system was identical to that of Example 1 exceptthat the solid polymer electrolyte was a polyurethane network made byreacting a triisocyanate with a low molecular weight (Mw=10,000)copolymer comprising 75% ethylene oxide and 25% methyl glycidyl ether.The characteristics of this device were:

maximum transmission changes at 550 nm: 35%→83%

number of cycles: over 3,000

times to achieve the transmission changes:

    ______________________________________                                        Transmission %:     83    35      76  38                                      Time (min.):         5    12       2   4                                      % of total transmission change:                                                                   100%          80%                                         ______________________________________                                    

EXAMPLE 3

The preparation of this system was identical to that of Example 1,except that the electrolyte was a liquid electrolyte made up with asolution of LiClO₄ (1M) in propylene carbonate and EC₂ was physicallyprepared. The two electrodes (Physical Ni based EC₂ and WO₃) wereassembled against each other with a plastic spacer in between and thesystem was then filled up by the liquid electrolyte.

The characteristics of this device were:

surface area: 20 cm²

working potential range: -0.8 V for coloring; +1.9 V for bleaching

charge (Li⁺) exchanged: 5 mC/cm²

number of cycles: over 250

maximum transmission changes: 10%→58%

times to achieve the transmission changes:

    ______________________________________                                        Transmission %:     58    10      46  12                                      Time (min.):        3.5   5       2.3 1.5                                     % of total transmission change:                                                                   100%          80%                                         ______________________________________                                    

EXAMPLE 4

The preparation of this system was identical to that of Example 1,except for the nature of the electrolyte.

In this Example, the Ni based EC₂ was an "electrochemically preparedEC₂." The solid polymer electrolyte was a "solid solution" of a lithiumsalt in a copolyether type polymer. The polymer was an ethylene oxidebased terpolymer comprised of: ##STR4##

The lithium salt was Li N(CF₃ SO₂)₂, incorporated in the polymer at 20%(wt.).

The characteristics of the devices were:

surface area: 20 cm²

working potential range: -1.6 V for coloring; +1.4 V for bleaching

charge (Li⁺) exchanged: 5 mC/cm²

number of cycles: over 300

maximum transmission changes: 40%→80%

times to achieve the transmission changes:

    ______________________________________                                        Transmission %:     80    40      65  33                                      Time (min.):         5    10       2   4                                      % of total transmission change:                                                                   100%          80%                                         ______________________________________                                    

EXAMPLE 5

The preparation of this system was identical to that of Example 1 exceptfor the transparent conductive electrodes, the Ni based EC₂ and thesolid polymer electrolyte. In this Example, the Ni based EC₂ was an"electrochemically prepared EC₂." The transparent conductive electrodeswere SnO₂ :F (fluorine doped tin oxide) prepared by chemical vapordeposition. The solid polymer electrolyte was a "solid solution" of alithium salt in a copolyether type polymer. The polymer was an ethyleneoxide based copolymer comprised of: ##STR5##

An equimolar mixture of LiClO₄ and LiN(CF₃ SO₂)₂ salt was dissolved inthe polymer (15% wt.) to form the solid polymer electrolyte.

The characteristics of the device were:

surface area: 20 cm²

working potential range: -1.6 V for coloring; +1.4 V for bleaching

charge (Li⁺) exchanged: 5-6 mC/cm²

number of cycles: over 300

maximum transmission changes: 37%→83%

times to achieve the transmission changes:

    ______________________________________                                        Transmission %:     83    37      78  41                                      Time (min.):         5    11       2   4                                      % of total transmission change:                                                                   100%          80%                                         ______________________________________                                    

The electrochromic element of the present invention can be useful inapplications such as for sun roofs of automobiles, architecturalwindows, aircraft windows, the rear windows of vans or trucks, or insunglasses. The electrochromic element of the invention can be utilizedto vary the amount of visible light transmitted through a substrate andalso can be utilized to reduce the amount of radiant heat transmittedthrough windows. Alternatively, the electrochromic element of theinvention can be utilized in a mirror to vary the percentage ofreflected visible light which would be useful, for example, in anautomobile rear view mirror.

Although the present invention has been described in connection with thepreferred embodiments, it is understood that modifications andvariations may be resorted to without departing from the spirit andscope of the invention. Such modifications are considered to be withinthe purview and scope of the invention and the appended claims.

What is claimed is:
 1. An electrochromic element comprising a pair ofconductive electrodes at least one of which is transparent, first andsecond inorganic electrochromic layers interposed between said pair ofconductive electrodes and an ion conducting layer of an electrolyteinterposed between said first and second inorganic electrochromiclayers, wherein said first and second inorganic electrochromic layersare different and are capable of exhibiting electrochromic propertiesupon the incorporation of an alkali metal or Ag ion, wherein theelectrochromic properties of said first and second inorganicelectrochromic layers are complementary, and wherein said ion conductinglayer of an electrolyte is a copolymer of ethylene oxide and methylglycidyl ether and at least one ionizable salt, or a copolymer ofethylene oxide and butylene oxide and at least one ionizable salt,capable of conducting an amount of alkali metal or silver ions betweensaid first and second inorganic electrochromic layers sufficient tocause each of said first and second inorganic electrochromic layers toexhibit a complementary color change, with said ion conducting layerremaining amorphous and transparent.
 2. The element of claim 1, whereinsaid ion conducting layer of a solid polymeric electrolyte comprises acopolymer of ethylene oxide and methyl glycidyl ether and at least oneionizable salt.
 3. The element of claim 1, wherein said ion conductinglayer of a solid polymeric electrolyte comprises a copolymer of ethyleneoxide and butylene oxide and at least one ionizable salt.
 4. The elementof claim 1, wherein said first inorganic electrochromic layer is WO₃. 5.The element of claim 1, wherein said first inorganic electrochromiclayer is MoO₃.
 6. The element of claim 1, wherein said first inorganicelectrochromic layer is CrO₂.
 7. The element of claim 1, wherein saidpair of conductive electrodes are fluorine doped tin oxide (SnO₂ :F). 8.The element of claim 1, wherein said pair of conductive electrodes areindium-tin oxide.
 9. An electrochromic element comprising a pair ofconductive electrodes at least one of which is transparent, first andsecond inorganic electrochromic layers interposed between said pair ofconductive electrodes and an ion conducting layer of an electrolyteinterposed between said first and second inorganic electrochromiclayers, wherein said first and second inorganic electrochromic layersare different and are capable of exhibiting electrochromic propertiesupon the incorporation of an alkali metal or Ag ion, wherein theelectrochromic properties of said first and second inorganicelectrochromic layers are complementary, and wherein said ion conductinglayer of an electrolyte is a polyurethane gel formed by reacting atriisocyanate with a copolymer of ethylene oxide and butylene oxidehaving a molecular weight ranging between 1,000 and 20,000, or acopolymer of ethylene oxide and methyl glycidyl ether having a molecularweight ranging between 1,000 and 20,000, and at least one ionizablesalt, capable of conducting an amount of alkali metal or silver ionsbetween said first and second inorganic electrochromic layers sufficientto cause each of said first and second inorganic electrochromic layersto exhibit a complementary color change, with said ion conducting layerremaining amorphous and transparent.
 10. The element of claim 9, whereinsaid first inorganic electrochromic layer is WO₃.
 11. The element ofclaim 9, wherein said first inorganic electrochromic layer is MoO₃. 12.The element of claim 9, wherein said first inorganic electrochromiclayer is CrO₂.
 13. The element of claim 9, wherein said pair ofconductive electrodes are fluorine doped tin oxide (SnO₂ :F).
 14. Theelement of claim 9, wherein said pair of conductive electrodes areindium-tin oxide.
 15. The element of claim 9, wherein said ionconducting layer of a solid polymeric electrolyte comprises:apolyurethane gel formed by reacting a triisocyanate with a copolymer ofethylene oxide and butylene oxide having a molecular weight rangingbetween 1,000 and 20,000; and at least one ionizable salt.
 16. Theelement of claim 9, wherein said ion conducting layer of a solidpolymeric electrolyte comprises:a polyurethane gel formed by reacting atriisocyanate with a copolymer of ethylene oxide and methyl glycidylether having a molecular weight ranging between 1,000 and 20,000; and atleast one ionizable salt.
 17. An electrochromic element comprising apair of conductive electrodes at least one of which is transparent,first and second inorganic electrochromic layers interposed between saidpair of conductive electrodes and an ion conducting layer of anelectrolyte interposed between said first and second inorganicelectrochromic layers, wherein said first and second inorganicelectrochromic layers are different and are capable of exhibitingelectrochromic properties upon the incorporation of an alkali metal orAg ion, wherein the electrochromic properties of said first and secondinorganic electrochromic layers are complementary, and wherein said ionconducting layer of an electrolyte is a polyurethane network resultingfrom the reaction of a copolymer of ethylene oxide and another cyclicoxide with an organic polyisocyanate coupling agent, capable ofconducting an amount of alkali metal or silver ions between said firstand second inorganic electrochromic layers sufficient to cause each ofsaid first and second inorganic electrochromic layers to exhibit acomplementary color change, with said ion conducting layer remainingamorphous and transparent.